|Don't have access to my spare carbs today (too many other things in the way) but I believe the screw is a standard American thread, either #8-32tpi or possibly #10-32tpi. These are common sizes that should be available at your local hardware store, Home Depot, etc.
Paraphrasing the Shop Manual, you can do an on-car test as described below. If you have a hand operated vacuum pump, then you can use it, rather than manifold vacuum, to test things, though you'll do a lot of pumping.
Using a tee fitting, place a vacuum gauge to monitor the intake manifold vacuum. Remove the hose connected to the smaller fitting on the vacuum switch, and connect a second vacuum gauge to this fitting. Start the engine, allow it to warm up, then increase its speed to about 4000 rpm. Decrease the throttle opening while monitoring both vacuum gauges. The switch must be open, applying vacuum to the second gauge, for manifold vacuums of 17.5 to 20.0" Hg or more, and must be closed, with no vacuum applied to the second gauge, for manifold vacuums of 15.5 to 16.5" Hg or less. Note that a particularly well-sealed vacuum switch may maintain considerable vacuum at its outlet side for several seconds. This does no harm, as bleed-down time is normally determined by the "calibrated leak" attached to the rear carburetor.
Removing or disabling the vacuum switch has the same effect as disabling the throttle bypass valves. Bypassing the vacuum switch, or an internal leak, can result in peculiar variations in engine idle speed.
As the vacuum switch is an emissions-related item, its permanent removal or alteration may be subject to local regulations.